This invention relates to optical fibers and in particular to a method for protecting a fiber surface by applying a seal. It is well known that bare uncoated fibers are susceptible to abrasion and corrosion. Various chemicals, including water, can react with a fiber damaging its optical properties and weakening its mechanical strength and static fatigue resistance. Microcracks in a fiber surface present regions susceptible to chemical attack, especially when the fiber is under stress. Fiber stress tends to open a crack, thereby focusing the strain onto the chemical bonds at the tip of the crack. These strained bonds are more easily broken thereby enabling corrosion to extend such microcracks. Growth of microcracks weakens the strength of a fiber producing static fatigue or sudden failure.
There presently exist a variety of coatings which protect a fiber from abrasion but not from corrosion. Because such coatings do not protect a fiber from corrosion, many prior art methods attempt to reduce microcrack degradation by employing expensive techniques to reduce the number of microcracks.Additionally, the use of fibers is often restricted to low stress applications. Another approach is to apply a metallic coating to a fiber to prevent water from reaching the fiber. It has been suggested to apply a metal seal of molten tin or aluminum which form a hermetic coating when cooled. However, metals tend to form polycrystalline solids which can themselves be rapidly corroded via greatly enhanced grain boundary diffusion. The relatively open structure of the grain boundaries provide an easy path for migrating ions to reach the SiO.sub.2 surface and nucleate and/or propagate cracks. Metal coats also provide an often undesirable electrical path along a fiber. In addition, many metals react with SiO.sub.2 to form metal oxides. There abrasive particles of metal oxides may roughen the SiO.sub.2 surface and act as stress raisers to provide easy nucleation sites for potential cracks.
Other inorganic materials, including silicon nitride, have been used in semiconductor technology for insulating, passivating and sealing devices and integrated circuits. (See, for example, M. T. Duffy et al., Preparation, Properties, and Applications of Chemically Vapor Deposited Silicon Nitride Films, RCA Review, December 1970 , pages 742-753.) However, these coatings have not been applied to optical fibers, in which the coating must be applied on-line at high speed in a continuous process. It should be noted that optical fibers are subjected to abrasion, bending and tensile strain not encountered by semiconductor substrates. For a fiber coating to be operational, it must therefore display properties (e.g., adhesion, fracture strength, microstructure, particle size, surface morphology), which are not required for semi-flat semiconductor coatings. In addition, the optical absorption of silicon nitride is higher than that of SiO.sub.2 which provides an advantageous characteristic of reducing optical crosstalk unique to optical fiber technology in certain configurations of multiple fiber bundles.